1. Field of the Invention
The present invention relates to a prestressing structure for rotationally balancing a motor. More particularly, the present invention relates to a motor rotor and a motor stator employing a balancing magnet and a magnetically conductive member to generate a prestressing force therebetween so that rotary balance of the motor is maintained.
2. Description of the Related Art
Referring initially to FIG. 1, Taiwanese Patent Publication No. 383818, entitled “HEAT DISSIPATION FAN STRUCTURE HAVING A MAGNETICALLY POSITIONING DEVICE,” discloses a conventional rotational balance structure for a motor. The motor includes a motor stator 10, an axial tube 20, a motor rotor 30 and a magnetically conductive metal plate 301. The motor stator 10 employs the axial tube 20 to mount on a hub base 100, and provides with a plurality of pole plates 11. The axial tube 20 accommodates a bearing member 21 that allows a rotary member. The motor rotor 30 consists of an axial shaft 31 and an annular magnet 32. In assembling, the axial shaft 31 of the motor rotor 30 extends through an elongated hole of the axial tube 30 and the annular magnet 32 of the motor rotor 30 corresponds pole faces of the pole plates 11 of the motor stator 10. The magnetically conductive metal plate 301 is attached to an inner surface of the motor rotor 30 in proper and combined therewith. Meanwhile, the magnetically conductive metal plate 301 has an outer circumference corresponding to top portions of the pole plates 11 of the motor stator 10. When the motor is operated, the pole plates 11 of the motor stator 10 magnetize the magnetically conductive metal plate 301 that may maintains rotary balance of the motor rotor 30. Consequently, it reduces operational vibration the motor and abrasion of the bearing member 21 that may increase the useful life of the motor.
However, there exist several drawbacks as follows:
1. The magnetically conductive metal plate 301 may result in reduction of excitation efficiency of the pole plates 11 due to magnetic loss of the magnetically conductive metal plate 301 with respect to the motor rotor 30.
2. The mass of the magnetically conductive metal plate 301 may also result in a heavy burden of the motor rotor 30 that may result in reduction of motor speed.
3. In starting the motor, the magnetically conductive metal plate 301 fails to provide with an attractive force on the motor stator 20 a short period for balancing rotary movement of the motor rotor 30. Consequently, it may result in abrasion of the bearing member 21 in long-term use.
Other Examples of such a rotational balance structure for a motor are illustrated by U.S. Pat. No. 6,097,120 to Homg, entitled “BRUSHLESS DC MOTOR ASSEMBLY,” Taiwanese Patent Publication No. 422365, entitled “HEAT DISSIPATION FAN STRUCTURE HAVING A MAGNETICALLY POSITIONING DEVICE,” and U.S. patent application Ser. No. 09/366,636, entitled “POSITIONING STRUCTURES FOR A MOTOR ROTOR.” In these patents, a magnetically conductive metal plate is mounted on a bottom portion of a motor stator and corresponding to an annular magnet of a motor rotor. An attractive force between the magnetically conductive metal plate and the annular magnet maintains rotational balance of the motor when operated. However, the magnetically conductive metal plate may result in reduction of excitation efficiency of pole plates of the motor stator which is in connection with the magnetically conductive metal plate. Moreover, the magnetically conductive metal plate may result in a short circuit of a printed circuit board which is in connection with the magnetically conductive metal plate.
Another conventional rotational balance structure for a motor, U.S. Pat. No. 6,483,209 to Homg et al., entitled “BALANCE RINGS FOR MOTORS,” it discloses a magnetically conductive metal plate mounted on a bottom portion of a motor stator. The magnetically conductive metal plate includes at least one upright wall corresponding to an annular magnet of a motor rotor. An attractive force between the upright wall of the magnetically conductive metal plate and the annular magnet maintains rotational balance of the motor when operated. However, the upright wall of the magnetically conductive metal plate may occupy a partial space for distributing blades of the motor rotor when the upright wall of the magnetically conductive metal plate is corresponding to an outer end circumference of the annular magnet. Alternatively, the upright wall of the magnetically conductive metal plate may result in reduction of excitation efficiency of pole plates of the motor stator when the upright wall of the magnetically conductive metal plate is corresponding to an inner circumference of the annular magnet. Consequently, the upright wall of the magnetically conductive metal plate may increase a total radius of the motor that may increase its dimensions of the motor.
Another conventional rotational balance structure for a motor, U.S. patent application Ser. No. 10/304,761, entitled “POSITIONING DEVICE FOR PRESTRESSING MAGNET OF SPINDLE MOTOR,” it discloses positioning means for mounting a prestressing magnet on a pole plate of motor stator in proper. The prestressing magnet attracts a metal casing of a motor rotor that balances rotary movement of the motor. However, the prestressing magnet is disposed proximate the pole plate of motor stator that may result in reduction of excitation efficiency of pole plate of the motor stator.
Another conventional rotational balance structure for a motor, U.S. Pat. No. 6,448,675 to Homg et al., entitled “ROTATIONALLY BALACING STRUCTURE FOR AN AC MOTOR,” it discloses an inner rotor type ac motor. The motor includes a balance plate and a permanent magnet ring each of which selectively mounted to distal ends of an outer stator and an inner rotor. An attractive force between the balance plate and the permanent magnet ring maintains rotational balance of the motor when operated. However, the prestressing magnet is disposed proximate the pole plate of motor stator that may result in reduction of excitation efficiency of pole plate of the motor stator.
The present invention intends to provide a motor rotor and a motor stator employing a balancing magnet and a magnetically conductive member to generate a prestressing force therebetween. The balancing magnet and the magnetically conductive member provide with an appropriate prestrssing force for balancing rotary movement and minimize a gap formed therebetween for prevent dust from entering therethrough. The positions of the balancing magnet and the magnetically conductive member are separated from pole plates of the motor stator to avoid magnetic loss in such a way to mitigate and overcome the above problem.